Diel phosphorus variation and the stoichiometry of ecosystem metabolism in a large spring‐fed river

Abstract

Abstract. Elemental cycles are coupled directly and indirectly to ecosystem metabolism at multiple time scales. Understanding coupling in lotic ecosystems has recently advanced through simultaneous high-frequency measurements of multiple solutes. Using hourly in situ measurements of soluble reactive phosphorus (SRP), specific conductance (SpC), and dissolved oxygen (DO), we estimated phosphorus (P) retention pathways and dynamics in a large (discharge, Q ' 7.5 m 3 /s) spring-fed river (Ichetucknee River, Florida, USA). Across eight multi-day deployments, highly regular diel SRP variation of 3-9 lg P/L (mean ;50 lg P/ L) was strongly correlated with DO variation, suggesting photosynthetic control directly via assimilation, and/or indirectly via geochemical reactions. Consistent afternoon SRP maxima and midnight minima suggest peak removal lags gross primary production (GPP) by ;8 hours. Two overlapping processes were evident, one dominant with maximum removal near midnight, the other smaller with maximum removal near midday. Hourly [Ca] measurements during three 24-hour deployments showed consistent afternoon minima, suggesting that calcite precipitates as GPP increases pH and mineral saturation state. Resulting P coprecipitation was modeled using SpC as a [Ca] proxy, yielding a diel P signal adjusted for the primary geochemical retention pathway. P assimilation, the dominant diel signal, was computed by interpolating between daily concentration maxima, both with and without geochemical adjustment, and converting concentration deficits to fluxes using discharge and benthic area. Adjusting for calcite co-precipitation yielded assimilation rates (13.7 6 5.8 mg PÁm À2 Ád À1) strongly correlated with GPP, accounting for 72% 6 9% of gross removal, and ecosystem C:P stoichiometry (466 [6 12]:1) consistent with dominant vascular autotrophs (478 [6 24]:1). Without adjusting for co-precipitation, covariance with GPP was weaker, and C:P implausibly high. Consistent downstream P accumulation, likely from P-rich porewater seepage, varied across deployments (3-22% of total flux) and co-varied with discharge and respiration. Asynchronous N and P assimilation may arise from differential timing of protein and ribosome production, with the latter requiring maximal carbohydrate stores and minimal photolytic interference. This study demonstrates direct and indirect coupling of biological, hydrological, and geochemical processes and the utility of high-resolution time series of multiple solutes for understanding these linkages.

Nutrient Dynamics of the Delta: Effects on Primary Producers

“…Collection of nutrient data at frequent intervals in aquatic systems has in almost all cases revealed much higher temporal variability than was evident in less frequent discrete sample collection (Bende-Michl et al 2013;Pellerin et al 2009Pellerin et al , 2011Pellerin et al , 2014Wild-Allen and Rayner 2014). These data also revealed patterns in nutrient dynamics that occur at yearly, seasonal, diurnal, tidal, and individual-event time-scales, which are difficult if not impossible to detect using lowerresolution data (Bende-Michl et al 2013;Bowes et al 2009;Cohen et al 2012Cohen et al , 2013Pellerin et al 2009Pellerin et al , 2011Pellerin et al , 2014Wild-Allen and Rayner 2014).…”

“…Although most high-frequency nutrient studies to date involve nitrate sensors, adoption of commercially available in situ analyzers for phosphate and prototype sensors for ammonium is growing (Rozemeijer et al 2010;Cassidy and Jordan 2011;Bende-Michl et al 2013;Cohen et al 2013;Gilbert et al 2013;Outram et al 2014;Bowes et al 2015). We are aware of few studies that report results from in situ high-frequency ammonium analyzers, and-not surprisingly-these studies found that ammonium, nitrate, and phosphate variability are not necessarily linked (Bende-Michl et al 2013;Gilbert et al 2013).…”

“…Daily dissolved oxygen patterns-when coupled with data on temperature, barometric pressure, and light regime-can be used to estimate gross primary production and ecosystem respiration in aquatic ecosystems as pioneered by Odum (1956) and now widely used in various aquatic ecosystems (Grace et al 2015). Metabolism estimates can also be coupled to continuous nutrient sensor data to examine both elemental stoichiometry and the coupling of primary production and ecosystem respiration to the uptake of nutrients such as ammonium, nitrate, and phosphate (Cohen et al 2013). Few places anywhere have the combined measurements of flows, fixed stations for basic water quality, and the network of highfrequency nutrient stations ( Table 1) that are found in the Delta.…”

Expanding the scale of aquatic sciences: the role of the National Ecological Observatory Network (NEON)

“…Advances in data collection using continuously monitoring sensors deployed on buoys or other stationary structures for water monitoring will allow researchers to circumvent some of the previous limitations of sampling large rivers and increase the temporal resolution of measurements (Cohen et al 2013). NEON is collaborating with the US Army Corps of Engineers in placing sensors at the Alabama River sites near USGS gauging stations for real-time discharge monitoring.…”

“…These factors make it difficult to link life-history traits with alterations in hydrologic variables without high-frequency measurements (Puckridge et al 1998). Cohen et al (2013) required very high temporal resolution of river data (on the scale of minutes) to distinguish between biotic and abiotic mechanisms of nutrient retention in a Florida river, suggesting that NEON's continuous monitoring on the Black Warrior-Tombigbee River system is a strong approach for documenting the influence of multiple variables on ecosystem processes.…”